This invention relates to a melt-flowable high-impact melt mixed blend of polypropylene and polyethylene, preferably high density polyethylene, believed to have an interpenetrating network structure which is stabilized against collapse during molding by inclusion of minor but effective amounts of an ethylene-propylene copolymer elastomer.
Ternary blends of low density polyethylene (LDPE), polypropylene (PP) and monoolefin polymer rubbers having improved impact strengths and optical clarity have been disclosed previously in U.S. Pat. Nos. 4,088,714 and 4,087,485, respectively. U.S. Pat. No. 4,088,714 discloses that the impact properties of polypropylene (PP) and cross linkable low density polyethylene (LDPE) blends could be significantly improved by addition of an ethylene-propylene copolymer elastomer (EPR) to the blend, in an amount such that the ratio of elastomer to the cross linkable low density polyethylene (LDPE) was approximately one to one, and curing the elastomer with the cross linkable LDPE to form a discontinuous, tightly cross linked phase of elastomer and LDPE which is intimately dispersed in a continuous phase of PP. Although a tightly cross linked phase of EPR and LDPE is not itself melt flowable, it was found that by being intimately dispersed in a continuous phase of PP which was melt flowable, the total blend assumed a benificial melt flowable characteristic. In U.S. Pat. No. 4,087,485 it was disclosed that an impact polypropylene blend having surprisingly good optical properties may be formulated by partially curing an EPR with a cross linkable LDPE in the presence of polypropylene, wherein the polypropylene component comprises about 70 to 95 percent of the total mixture.
Although such blends of PP and LDPE had an excellent balance of overall mechanical properties as compared to previous PP/LDPE blends, this improvement depended upon the achievement of crosslinking of the EPR with the LDPE. Additionally, the employment of a LDPE does not permit the achievement of as high a tensile strength, flexural modulus or impact strength in the final blend as could be achieved if a high density polyethylene (HDPE) could be used.
Certain binary blends of HDPE and PP have been previously formulated which have a good balance of low temperature impact strength (Izod) and flexural modulus. Such blends, which exhibit these good mechanical properties when the relative concentrations of PP to HDPE is about 1:1 to 2:1, have proved unsatisfactory for molding large finished parts wherein the molding process employs high temperatures and pressures, e.g. in injection molding. Parts molded from such binary blends of PP/HDPE have failed due to delamination of the two phases. Additionally, although binary blends of PP and HDPE have good Izod impact and flexural modulus properties, it has been found that their Gardner impact strengths are exceptionally poor.
Studies of the morphology of melt mixed binary blends of PP and HDPE, wherein the relative ratio of PP/HDPE is about 1:1 to about 2:1, indicate that upon melt mixing an interconnected three dimensional continuum network of HDPE interlocked with an interconnected three dimensional continuum network of PP, a dual continuum structure, may be formed. This interpenetrating interlocked network structure is believed to account for the good Izod impact-flexural properties observed in parts molded from such blends, but is relatively unstable under shear in the melt state thereby providing for a good melt rheology.
The delamination observed in finished parts which have been molded from such PP/HDPE binary blends is believed to stem from a disruption of this dual continuum network structure which occurs in the molding process. Heat and pressure encountered in the molding process causes the interlocking dual continuum network structure to collapse. Upon collapse the blend structure is believed to assume a machine oriented fibrous tape-like dispersion wherein a multiplicity of tape-like structure of HDPE and PP become interlayered, one upon the other, within the molded part. Molding processes convert the internal structure of melt mixed blend from that of a interlocked dual continuum to that of machine oriented separate layers along which, due to the poor interfacial bonding which exist between HDPE and PP, surface delamination may occur in finished molded parts, presumably occuring along the HDPE/PP interface.
Previous workers have included monoolefin polymer rubbers, e.g. ethylene-propylene copolymer elastomers (EPR) and the like in certain binary blends of HDPE and PP to improve the mechanical properties of such blends. Blends of monoolefin polymer rubbers, e.g., ethylene-propylene copolymer elastomers and ethylene, propylene and copolymerizable polyene terpolymer elastomers, with polyolefins, e.g., high density polyethylene, polypropylene and the like, which can be processed and fabricated by methods used for thermoplastics and have elastomeric properties without requiring vulcanization are well-known. Furthermore, thermoplastic elastomer blends of partially cured monoolefin copolymer rubbers and certain polyolefins are known. See, for example, U.S. Pat. Nos. 3,758,642 and 3,806,588. It is disclosed in both of these prior art patents that the partial curing of the monoolefin copolymer rubber is essential to produce a blend which has the characteristics of a thermoplastic resin, i.e., which can be reprocessed, while also having elastomeric characteristics.
Known thermoplastic elastomer blends of monoolefin polymer rubbers and polyolefin resins suffer from a disadvantage of having less than desirable overall balance of mechanical and physical properties, such as low resiliency, tensile strength, stiffness, surface hardness, and/or high heat distortion, permanent or tension set, etc. Furthermore, many known blends, including blends prepared in accordance with the teachings of the above-mentioned U.S. Pat. Nos. 3,758,643 and 3,806,558 or U.S. Pat. No. 3,835,201, have less than desirable melt rheologies, e.g., high viscosity at high shear rates and the high melt temperatures normally used in injection molding. Such undesirable characteristics restrict the use of such known thermoplastic elastomer blends in the manufacture of many types of flexible molded or extruded articles. This is particularly evident in the manufacture of flexible body components for the automotive industry. For such a use, a thermoplastic elastomer is required which has such characteristics as relatively low viscosity at high shear rates at melt temperatures for use in high-speed injection molding or extrusion techniques to provide flexible body components having high resiliency, tensile strength, flexural modulus, etc. along with low permanent set, heat distortion and the like.
The addition of EPR to certain PP/HDPE blends has also been proposed to overcome certain processing problems. U.S. Pat. No. 3,256,366 teaches a method whereby a vulcanizing agent, such as a peroxide may be intimately mixed with a high or low density PE, without encountering the prevulcanization problems inherent in such mixing, by premixing the PE with between 10% to 70% by weight, of a rubbery copolymer such as EPR which acts as a diluent. U.S. Pat. No. 3,256,366 additionally discloses that up to 30% of PP may be mixed with the PE/EPR blend to improve the mechanical properties of a finished vulcanized article formed therefrom.
A recent effort to develop a low EPR content PP and PE blend with improved impact resistance is dislosed in German Offenlegungsschriften No. 2,801,217 to Mitsui. Mitsui comprises a three step polymerization process wherein isotactic PP is prepare is the first step. In a second step ethylene-propylene is polymerized in the presence of the PP of step one, forming a PP-EPR block copolymer. Finally, PE is prepared in a third step, in the presence of the PP-EPR of step two, to produce a finished high impact block copolymer blend comprising about 35.2 mol.% PE, 4.85 mol.% amorphous polymer (EPR) and the remainder (59.95 mol.%) PP. From Mitsui it appears that block copolymerization is required to produce a high impact formulation with a good balance of other mechanical properties.
U.S. Pat. No. 3,256,367 discloses that the impact strengths, stiffness and heat resistance properties of PP/EPR blends may be improved by the addition of a HDPE component in a PP:HDPE ratio of from about 2:1 to about 48:1 and preferably 3.25:1 to 23:1. The combined amount of EPR and HDPE is preferably maintained at less than 35% by weight in order to produce a mixture of desirable flexural characteristics. The ratio of EPR/HDPE is generally one or greater. At a EPR/HDPE ratios of about 1.0 or greater, the flexural modulus of such mixtures falls off rapidly as the concentration of combined EPR and HDPE increases, especially as it exceeds 33%. Yet, when an EPR/HDPE ratio of less than one is employed the 0.degree. C. Izod impact strength is significantly worse. The rapid loss in low temperature impact strengths which occur at EPR/HDPE ratios less than one requires, in order to maintain good low temperature impact properties, the employment of the more expensive EPR in order to keep the EPR/HDPE ratio greater than one.
It is desirable in impact polypropylene formulations to achieve a high level of low temperature impact strength while maintaining a high room temperature flexural modulus. Such combination of properties are exhibited by melt mixed blends of PP and HDPE wherein the respective ratio of such components is from about 1:1 to about 2:1. However, such blends alone cannot be used to produce satisfactory molded parts because of the occurence of delamination in such parts and because falling weight, low temperature impact strength is unacceptably poor.